1. Technical Field
The present invention relates to a contact for a fluid level detection apparatus and a fluid level detection apparatus.
2. Background Art
FIGS. 5 to 7 show a conventional fluid level detection apparatus.
A fluid level detection apparatus 1 shown in FIGS. 5 to 7 is disclosed in JP-A-2009-8535. The fluid level detection apparatus 1 includes a frame 3 which is fixed within a vessel which stores a fluid whose level is to be detected, a holder 5 which is supported on the frame 3 so as to rotate in response to a change in fluid level within the vessel, a contact 7 which is fixed to the holder 5, a resistor plate 9 which is fixedly provided on the frame 3 and output terminals 11a, 11b which output a signal indicating a level of the fluid within the vessel.
As is shown in FIG. 5, the holder 5 is pivotally and rotatably attached to a rotating fulcrum 13 on the frame unit 3. A proximal end of a float arm 15, which has a float attached to a distal end thereof, is fixed to the holder 5. The holder 5 rotates about the rotating fulcrum 13 by virtue of a rotating moment M1 (refer to FIG. 5) which is applied to the float arm 15 by buoyancy acting on the float.
As is shown in FIG. 6, the contact 7 includes a contact support spring 17 made of a conductive material which is fixed to the holder 5 in a cantilevered fashion so as to rotate together with the holder 5 and a first contact 18 and a second contact 19 which are made of a conductive material and which are attached to a free end side of the contact support spring 17.
As is shown in FIG. 7, the contact support spring 17 includes a first support spring 21, a second support spring 22, and a connecting portion 23 which connects the first support spring 21 and the second support spring 22 together at fixed ends (proximal ends) thereof. At a free end (distal end) of the first support spring 21 is set a first position 21a to which the first contact 18 is attached. At a free end (distal end) of the second support spring 22 is set a second position 22a to which the second contact 19 is attached. Consequently, the first support spring 21 and the second support spring 22 are integrated with each other at the proximal ends thereof for electrical conduction.
As is shown in FIG. 6, the contact support spring 17 is supported on the holder 5 in the cantilevered fashion by the connecting portion 23 being fixed to the holder 5.
The first support spring 21 which constitutes the contact support spring 17 includes a pair of right and left plate springs 21b, 21c which are connected together at proximal ends thereof by the connecting portion 23. The pair of right and left plate springs 21b, 21c are connected together at free ends thereof, and the first position 21a is set at a junction 21d where the pair of right and left plate springs 21b, 21c are so connected. The first support spring 21 functions as a single cantilever when a bending load is applied to the first position 21a set at the free end thereof.
As is shown in FIGS. 7 and 8, either of the pair of plate springs 21b, 21c which constitute the first support spring 21 functions as a cantilever having a uniform rectangular section whose width and height dimensions are expressed as “b” and “h”, respectively, and a length expressed as “L2.”
The second support spring 22 also has a similar configuration to that of the first support spring 21. Namely, the second support spring 22 includes a pair of right and left plate springs 22b, 22c which are connected together at proximal ends thereof by the connecting portion 23. The pair of right and left plate springs 22b, 22c are connected together at free ends thereof, and the second position 22a is set at a junction 22d where the pair of right and left plate springs 22b, 22c are so connected. The second support spring 21 functions as a single cantilever when a bending load is applied to the second position 22a set at the free end thereof.
As is shown in FIGS. 7 and 8, either of the pair of plate springs 22b, 22c which constitute the second support spring 22 functions as a cantilever having a uniform rectangular section whose width and height dimensions are expressed as “b” and “h”, respectively, and a length expressed as “L1.”
As is shown in FIGS. 6 and 7, the resistor plate 9 includes a first sliding resistor 25 and a second sliding resistor 26 which are fixedly attached to the resistor plate 9.
The first sliding resistor 25 is attached to the resistor plate 9 so that the first contact 18 is brought into press contact with the first sliding resistor 25 with the contact support spring 17, as is shown in FIG. 6, put in a state in which the free end thereof is flexed to be displaced in a direction indicated by an arrow B in FIG. 6 (to be exact, the free end of the first support spring 21 is flexed to be displaced in the direction indicated by the arrow B in FIG. 6). In addition, the first sliding resistor 25 is formed into an arc shape so that the first contact 18 is allowed to slide thereon when the holder 5 rotates.
On the other hand, the second sliding resistor 26 is attached to the resistor plate 9 so that the second contact 19 is brought into press contact with the second sliding resistor 26 with the contact support spring 17, as is shown in FIG. 6, put in the state in which the free end thereof is flexed to be displaced in the direction indicated by the arrow B in FIG. 6 (to be exact, the free end of the second support spring 22 is flexed to be displaced in the direction indicated by the arrow B in FIG. 6). In addition, the second sliding resistor 26 is formed into an arc shape so that the second contact 19 is allowed to slide thereon when the holder 5 rotates.
One end side of the first sliding resistor 25 is electrically connected to the output terminal 11a. In addition, one end side of the second sliding resistor 26 is electrically connected to the output terminal 11b. 
The contact 7 electrically connects the contact portion of the first contact 18 with the first sliding resistor 25 and the contact portion of the second contact 19 with the second sliding resistor 26 via the contact support spring 17. Consequently, the first sliding resistor 25 and the second sliding resistor 26 are electrically connected at the connecting portions where the contact 7 is in contact therewith.
In other words, in the fluid level detection apparatus 1, when the holder 5 rotates in response to a change in level of the fluid in the vessel, the contact positions of the contact 7 with the first sliding resistor 25 and the second sliding resistor 26 are changed by the change in fluid level, whereby a resistance value between the output terminals 11a, 11b is changed. Thus, the fluid level detection apparatus 1 utilizes the resistance value between the output terminals 11a, 11b as a signal indicating the level of the fluid in the vessel.
FIG. 9 shows a cantilever 28 having a length expressed as “L” which constitutes a basic model for designing the contact 7.
Assume that the cantilever 28 has a uniform cross-section whose width and height dimensions are expressed, respectively, as “b” and “h” and a Young's modulus expressed as “E.”
The second moment I of area of the cantilever 28 is expressed by the following expression (1).I=bh3/12  (1)
A relationship between y and W, where a flexure displacement y is generated at a free end of the cantilever 28 by a pressing load W under which the sliding resistors 25, 26 are brought into press contact with the free end is expressed by the following expression (2).y=WL3/(3EI)  (2)
When the expression (2) is substituted by the expression (1), the load W applied to the free end is expressed by the following expression (3).W=yEbh3/(4L3)  (3)
Since, the pair of right and left plate springs 21b, 21c in the first support spring 21 and the pair of right and left plate springs 22b, 22cin the second support spring 22 are simple beams whose width and height dimensions are b and h, respectively, a load W1 under which the flexure displacement y is generated in the first position 21a of the first support spring 21 becomes twice the load obtained by the expression (3) above and is expressed as below.
                                                                        W                ⁢                                                                  ⁢                1                            =                              2                ⁢                                  {                                      y                    ⁢                                                                                  ⁢                    E                    ⁢                                                                                  ⁢                    b                    ⁢                                                                                  ⁢                                                                  h                        3                                            /                                              [                                                  4                          ⁢                                                                                    (                                                              L                                ⁢                                                                                                                                  ⁢                                2                                                            )                                                        3                                                                          ]                                                                              }                                                                                                        =                              y                ⁢                                                                  ⁢                E                ⁢                                                                  ⁢                b                ⁢                                                                  ⁢                                                      h                    3                                    /                                      {                                          2                      ⁢                                                                        (                                                      L                            ⁢                                                                                                                  ⁢                            2                                                    )                                                3                                                              }                                                                                                          (        4        )            
Similarly, a load W2 under which the flexure displacement y is generated in the second position 22a of the second support spring 22 is expressed as below.W2=yEbh3/{2(L1)3}  (5)
When designing the contact 7, the following conditions (a) and (b) are required to be met.
(a) In order to ensure reliability in electrical connection between the sliding resistors 25, 26 and the contact support spring 17, in the first support spring 21 and the second support spring 22, the flexure displacement y resulting when the free end sides of the first and second support springs 21, 22 are brought into press contact with the corresponding sliding resistors 25, 26 is set to a certain magnitude or larger.(b) The sectional dimensions (b and h) and the lengths L1, L2 of the first support spring 21 and the second support spring 22 are set so as to ensure that the load W which generates the flexure displacement y set under (a) is applied to the first support spring 21 and the second support spring 22 and that the first and second support springs 21, 22 have a strength which enables them to bear vibrations applied thereto from the periphery.
In the contact 7 disclosed in JP-A-2009-8535, however, as is shown in FIG. 7, the first contact 18 and the second contact 19 are constructed so as to be supported, respectively, by the corresponding support springs 21, 22 which are provided exclusively to support them. Because of this, for example, in the event that the sectional dimensions and the length L1 of the second support spring 22 which is disposed further inwards are designed to meet the design conditions (a) and (b) described above, the first support spring 21 disposed to surround the outside of the second support spring 22 has to be designed to become larger in size than the second support spring 22, causing a problem that the contact 7 is enlarged in size, which then triggers another problem that an increase in size of the fluid level detection apparatus 1 is called for.
Further, in the contact 7 of JP-A-2009-8535, since the first contact 18 and the second contact 19 are supported individually by the corresponding support springs 21, 22 which are provided exclusively to support them, the loads W1, W2 (refer to the expressions (4), (5) above) under which the flexure displacement y is generated at the free ends of the support springs 21, 22 become entirely pressing loads applied to the contacts 18, 19, respectively, whereby the pressing loads applied to the respective contacts 18, 19 become too high. Therefore, since the wear of the contacts resulting from sliding is promoted, the lives of the contacts are shortened, leading to a problem that it is difficult to enhance the durability of the fluid level detection apparatus 1.